The invention concerns a security document comprising a translucent carrier substrate, in particular of paper and/or plastic material, and at least one security element which is applied to the carrier substrate or embedded in the carrier substrate and which presents at least one image when viewed in the transillumination mode from at least a first side of the security document and simulates a presence of at least a first watermark in the carrier substrate, wherein the security element has at least region-wise at least one layer which simulates the at least one first watermark and which locally alters the visually perceptible translucence of the carrier substrate.
Such security documents are known from WO 99/13157 A1. Here, a security film is applied to or integrated into a value-bearing paper or bond, as a security element. The security film comprises a translucent carrier film and a metallic coating which applied thereto and which has metal-free regions which are to be clearly recognised in particular in a transillumination mode. The metallic coating is divided into individual raster points producing a half-tone image. If the security film is embedded between two layers of a security paper, the presence of a watermark is simulated in the security paper by the metallic coating, and the watermark can be clearly perceived in the transillumination mode.
A conventional watermark in paper is produced by the thickness of the paper being locally altered in the manufacture thereof so that there are differences in transmission in the paper. In the transillumination mode, a continuous grey scale image, referred to as the watermark, can be perceived by a viewer from both sides of the paper.
Simulation of a watermark by a security element has the advantage that the complicated and expensive production process, as is required with conventional watermark formation on paper substrates, can be avoided. In addition, by means of a simulated watermark, it is also possible for a translucent plastic substrate to be easily provided with a watermark effect. Only embedding or applying a security element formed independently of the translucent carrier substrate of the security document, in or to the translucent carrier substrate, whether now it is of paper, plastic material, or also Teslin®, or laminates of those materials, is required. In that respect, depending on the respective configuration of the security element, a wide range of different watermarks can be simulated in one and the same carrier substrate.
It has been found however that the simulation of watermarks by means of separate security elements on a security document can also be performed by a forger at a viable level of complication and effort. For that purpose for example an imprint is arranged or a mask layer is glued in place between the paper layers to simulate the desired grey scale image.